In: Gecseg, F.(ed.): Lecture Notes in Computer Science, Vol. 117: Proc. of the 1981 Int. FCT-Conf., Szeged, Hungary, pages 350-359. Berlin: Springer-Verlag, 1981.
Also in: 9 (Full Version): Elektr. Informationsverarbeitung und Kybernetik EIK 17, 8, pages 389-416. 1981.